1. Field of the Invention
The present invention relates to systems for monitoring sterilant vapor concentration and more particularly, to a system for monitoring a condensable sterilant vapor in the presence of another condensable vapor.
2. Description of the Prior Art
In 1979 Moore et al. disclosed in U.S. Pat. No. 4,169,123 a method of sterilizing with hydrogen peroxide vapor. Forstrom et al. U.S. Pat. No. 4,169,124 disclosed a similar method wherein the concentration of the hydrogen peroxide vapor is less than 75 mg/L. Bier U.S. Pat. No. 4,642,165 discloses a method of vaporizing multicomponent liquids such as hydrogen peroxide and water, and passing the vapor into a sterilization chamber. Heretofore, no simple, unambiguous means of measuring hydrogen peroxide vapor concentration in the sterilization chamber has been available.
Hydrogen peroxide is rarely, if ever, used in a pure form. It is typically mixed with water. Hydrogen peroxide vapor, like water vapor, is a consensable gas, but is less volatile than water. When injected into a sterilizer along with water vapor, the hydrogen peroxide vapor may readily decompose into water and oxygen or condense into liquid. General physical properties, such as pressure, temperature and mass are therefore, not alone sufficient to provide an unambiguous measure of instantaneous hydrogen peroxide concentration in a sterilizer. Because of the potential for degradation of the sterilant, monitoring the hydrogen peroxide concentration is important to ascertain whether a sufficient sterilant concentration is maintained long enough to effect sterilization.
Known methods for determining hydrogen peroxide concentrations are not convenient for vapor phase hydrogen peroxide (VPHP) in a sterilizer because of sampling difficulties. Withdrawing a representative sample from a sterilizer is fraught with uncertainty because of the potential for degradation on surfaces (such as syringe needles), condensation, and, in the case of a vacuum sterilizer, because of a negative pressure gradient of up to 14 psi or more. A sample of vapor may be trapped in liquid water and subsequently analyzed for hydrogen peroxide by one of many known chemical or instrumental methods. Direct measurement of the VPHP may be accomplished by spectroscopic means with a very long (eg., 30 meters) cell and appropriate signal enhancement. Again, sampling the vapor is a problem. To overcome the vapor sampling difficulties, the spectroscopic cell might be mounted within the sterilizer chamber and means provided to transmit radiation through the chamber wall. These methods are cumbersome, expensive, and slow.
There is a need for a simple system to determine the vapor concentration of a condensable vapor of interest. There is a further need for a system to monitor sterilant vapor concentration. Finally, there is a further need for a system that will consistently provide such determinations quickly and accurately throughout the course of a sterilization cycle.